Coming from a non-SLR background I am at the moment very reluctant to resort to a tripod.

You shouldn't need to (most of the time).

I am also not very good at avoiding camera shake when handheld at much below 1/90th of a second.

Join the club. Actually, I'm getting pretty decent at minimizing camera shake during slow shutter speeds when I don't have a tri/monopod handy. I took a tip from winter biathletes (you know, those nutters that go around xcountry skiing and target shooting) -- breath slow, take the picture after you've exhaled, and after your heartbeat (I've got about 1/2 second to a full second between heartbeats, depending on how much activity I've been doing, which is plenty of time). In addition to that, stabilize the camera as much as possible by tucking your elbows into your chest -- don't let them flap in the wind. If you slouch a bit, you can even use your tummy as a bit of a rest for your elbows (this may be easier for some people than others ). Of course, if there's something to lean against, lean against it -- use your body as the tripod.

How much would a monopod help me ? (I think I'd be more likely to use a monopod than a tripod)

Probably a bit in this case. One option that you could try is a small tripod that you can double as a monopod by just extending one leg. I have a small Gitzo tripod that I bought for my recent trip to Costa Rica, which I used as a monopod on occasion. It's not much bigger than my monopod, and it gave me more flexibility than just having a monopod.

P.S. I know I have a lot to learn, for instance it has occured to me that all my shots today were at 100 ISO, so I guess I could have helped myself by using 200 or 400 ISO.

That would have helped.